How to Replace a T12 Ballast with a T8

A pair of fluorescent lights in a tiled ceiling.
  • 1 hours
  • Beginner
  • 40
What You'll Need
T8 ballast
Wire nuts
Screw driver
Wire cutters
Wire stripper
T8 bulbs

When your fluorescent lights stop working completely or flicker, then you may have to fix or change the ballast. The ballast limits the current passing through the lamp so that you get just the right amount of light that is necessary. A ballast can last for years without ever needing repair, but eventually, they will deteriorate and will need to be taken care of.

It is important to note that T12 type ballasts have been discontinued due to poor efficiency and are actually being phased out in favor of the T8 variety, so replacement is the recommended procedure when dealing with a bad T12.

Step 1 – Identifying the Ballast and Tubes

Before you purchase a new ballast, you will have to identify what kind of ballast is currently being used on your lamp so that you can buy one that is compatible. On your lamp, look for the identification code. It should be marked F(number)T(number). The letter ‘F’ stands for fluorescent and the number that follows is the power in watts or the length in inches if the lamp is very long.

The letter ‘T’ means that the bulb is a tubular shape. Confirm that you have a T12 fixture rather than something else. Along with the T12, the T8 is also a standard size. Another way to differentiate the tubes is by knowing the diameter for each. The diameter for the T12 is 38mm and 25mm for the T8.

Step 2 – Buy Ballast

Now that you have made sure that you have a T12 ballast, which can be replaced with a T8, you can purchase them from a home improvement store in order to replace the old ballast.

Step 3 – Turn Off Power

Before you take out the old ballast, you should turn off the power to the light. Do this by putting the switch in the off position and then turning off the power from the circuit breaker. Since you will not have light from the lamp at that time, you should replace the ballast during the day when there is natural light coming into the room from the sun. You may also want to use a flashlight if necessary.

Step 4 – Remove Diffuser Shield

You should see a plastic diffuser shield, which is used to help spread the light. Once located, remove this shield and any tubes inside.

Step 5 – Disconnect Wires

First, locate pairs of black and white wires coming through the fixture from the ceiling. Remove the wire nuts and disconnect the wires. Then, replace the wire nuts on the ends of the pair coming from the ceiling so that in the event that some accidentally turns the power back on, you’re not in any danger of accidental shock.

Next, remove all the wires connecting the ballasts to the quick connects, or tombstones, at the end of the fixture, and unscrew the holding nut from the ballast to remove it completely

If the wiring does not have any wire nuts, then you will have to cut the wires instead of just disconnecting them. Cut them about 12 inches from the middle of the fixture.

Step 6 – Replace the Old with the New

Before you connect the new T8 ballast in place of the T12, you’ll want to add a jumper, connecting the outside ports on the tombstones on one end of the fixture. For a two-bulb fixture, you’ll only need one jumper total, but for a four, you will need two, one connecting the outside ports on one pair of consecutive tombstones, and then one for the other set.

Mount the new T8 ballast in place and then start connecting the wires. In a four-bulb fixture, one side will connect two red wires and two blue wires to the four different tombstones; two red next to one another and two blue on the remaining pair. As for the other end, you will just connect the two yellow wires each to an open slot on one of the sockets connected by the jumper.

With this done, all you have to do is connect the black and white wires on the ballast to those coming from the ceiling with wire nuts.

The same basic principle of installation will hold true for a two-bulb fixture, just with half as many wires to connect to the bulb sockets.

Step 7 – Finish Up and Test

Put new T8 tubes in the sockets and put the diffuser into the fixture. Turn on the power to the circuit breaker and turn the switch to the on position to check if your fluorescent light is working properly.